The present invention relates to methods and apparatus for converting coal, air and high temperature steam into three separate gas streams—one consisting of wet, substantially pure hydrogen, a second containing “sequestration-ready” carbon dioxide, i.e., CO2 that is relatively pure and is at an elevated pressure thereby rendering its disposal less difficult, and a third stream consisting of oxygen depleted air.
More particularly, the invention relates to a process in which mixtures of coal, calcium and iron compounds are circulated among multiple reactors charged with either high temperature steam or compressed air that produce essentially pure hydrogen for use in fuel cells as a product of a controlled gasification reaction. The process according to the invention results in a separable and substantially pure carbon dioxide waste stream having residual amounts of sulfur dioxide, and an oxygen depleted air stream having high temperature heat that can be used, for example, in downstream power generation subsystems. The oxidation/reduction reactions of the present invention are much more thermodynamically efficient than conventional fossil fuel mixed combustion systems and offer significant environmental advantages over prior art processes using coal or other fossil fuels or biomass fuels to generate heat and combustion gases for use in gas turbine engines.
During the 21st century, the United States will continue to rely heavily on fossil fuels, such as natural gas, oil and petroleum distillates, as the primary source of fuel for gas turbine engines used to generate electrical power. Recently, the use of substantially pure hydrogen in fuel cells has been found to be more efficient and virtually pollution-free as compared to other conventional fossil fuel/air combustion technologies. Hydrogen fuel cells would be an ideal solution to many of the nation's energy needs as a clean-burning fuel source. However, the need exists for a thermodynamically-efficient and economical process capable of producing large amounts of pure hydrogen from a readily available and inexpensive energy resource such as coal.
Various conventional systems exist for oxidizing (burning) coal to generate free hydrogen in addition to producing heat for generating steam. Invariably, such systems pose significant environmental problems because of the potential release of oxidized carbon and sulfur compounds into the atmosphere from burning coal. Conventional hydrogen generating methods also involve high equipment costs due to the inefficiencies inherent in attempting to recover and isolate hydrogen from the other products of fossil fuel/air combustion.
It is also well known that the carbon dioxide resulting from coal-fired systems contributes to the greenhouse effect in the atmosphere and potential global warming. Other types of air pollution produced by coal combustion include particulate emissions, such as fine particles of ash resulting from pulverized coal firing, as well as the release of undesirable oxides of nitrogen, chiefly NO and NO2.
Thus, a significant need exists to produce relatively pure free hydrogen for use in electrical power generation in an economical and thermodynamically efficient manner, but without polluting the atmosphere. The need also exists to control the nature and extent of any carbon dioxide, and sulfur dioxide emissions created during coal combustion by isolating and disposing of the oxidized contaminants without releasing them into the atmosphere. Ideally, coal and other fossil fuels could be used to generate heat in a manner that allows the by-products of combustion, particularly CO2, to be readily and economically recovered at elevated pressure and in a relatively pure state, i.e., making the CO2 “sequestration-ready.”
In the past, a number of different CO2 disposal methods have been proposed such as pumping liquid CO2 into deep parts of the ocean. However, one recurring problem in the disposal of CO2 concerns the purity of the waste stream itself. Since most disposal options involve liquid CO2, it is generally accepted that for CO2 to be “sequestration-ready,” it cannot contain more than small amounts of impurities or other gases that do not liquefy under pressure.
In addition to air pollution problems, the combustion of coal to drive gas turbine engines suffers from the same limitations in thermodynamic efficiency inherent in all systems that rely on mixed (air) combustion of coal as the primary heat source. Gas turbines are considered to be among the lowest capital cost systems available for generating electrical power. However, their thermodynamic efficiency is notably lower than other systems. Although the efficiency increases with increasing turbine inlet temperature, the hot gases produced by coal firing contain fly ash which can be erosive to turbine blades. The higher temperature exhaust vapors can also be corrosive because of the acidic by-products of coal combustion, such as sulfur dioxide and HCl. Consequently, the maximum turbine inlet temperature that can be tolerated for coal firing is considerably lower than that associated with a “clean” fuel, such as oil or natural gas.
Over the years, some improvements in gas turbine metallurgy have increased the inlet temperatures that could be tolerated with coal-fired systems. By definition, the same technological advances serve to increase the inlet temperatures for cleaner fuels such as natural gas. Thus, the disadvantages of coal relative to cleaner fuels remain regardless of the gas turbine metallurgy involved and prevent coal despite its lower cost from being considered an attractive gas turbine fuel. The gas turbine industry has long recognized that if a process could be developed for burning coal in a manner that produced large quantities of relatively “clean” hot gases that were not erosive or corrosive, coal could become a much more economically viable fuel source for use in electrical power generation.
One proposed solution to the problem of using coal to power gas turbines is a process known as “gasification” in which coal and steam are fed to a high temperature reactor vessel and react to form a mixture of H2, CO and CO2. Because the gasification reaction is endothermic, heat must be supplied in some manner. Thus, in most gasification designs, air is mixed with the high temperature steam so that a portion of the coal burns while the remainder reacts with steam to form H2, CO and CO2. In other designs, a portion of the fuel solids are heated by combustion and then mixed with coal and steam to supply the heat needed to drive the gasification reaction forward.
The literature describes a coal gasification process in which a CO2 acceptor (either limestone or dolomite) circulates between a pair of fluid beds, one fluidized with steam and the other with air. See G. P. Curran, C. E. Fink, and E. Gorin (Chapter 10 in FUEL GASIFICATION, ACS Advances in Chemistry series 69, 1967). The temperature in the steam-fluidized bed remains low enough so that the CaO+CO2=CaCO3 reaction gasifies coal to virtually pure hydrogen. Only part of the carbon in the coal, however, becomes gasified in the steam fluidized reactor. The remainder moves to an air fluidized bed where it is oxidized (“burned”), liberating heat and decomposing the CaCO3 back into CaO. Since the CO is in equilibrium with the CO2 via the well-known water gas shift reaction, removal of the latter removes the former. The basic gasification process has the advantage of producing relatively pure hydrogen, but suffers from a disadvantage in that the CO2 is released directly into the atmosphere along with air and other oxidized by products of coal combustion such as sulfur dioxide.
U.S. Pat. Nos. 5,339,754; 5,509,362; and 5,827,496 (incorporated herein by reference) disclose a method for burning fuels using a catalyst that can be readily reduced when in an oxidized state, and then readily oxidized when in a reduced state. The fuel and air are alternately contacted with the catalyst. The fuel reduces the catalyst and is oxidized to CO2 and water vapor. Thereafter, the air oxidizes the catalyst and is depleted of oxygen. Thus, combustion is effected without the need to mix the fuel and air either prior to or during the combustion process. If means are provided whereby the CO2, water vapor and oxygen-depleted air are directed in different directions as they leave the combustion process, mixing can be completely avoided. This later method of combustion has been called “unmixed combustion.”
The total volume of combustion gases produced by unmixed combustion is comparable to that produced in conventional combustion, but with one significant difference. The volume of the CO2+water vapor steam represents only a small part of the total. As those skilled in the art will appreciate, the cost of removing acid gases from combustion effluents by scrubbing increases with the volume of gas being scrubbed. Thus, if unmixed combustion can be accomplished such that the acid gases leave the combustion process in the form of a CO2+ water vapor steam, the volume of gas that must be scrubbed can be substantially reduced, with a commensurate lower operating cost. As detailed below, operating unmixed combustion in a manner such that the acid gases leave the combustor in the CO2+ water vapor steam requires an appropriate choice of catalyst and close control over the initial combustion reaction and subsequent decomposition reaction.
The subject matter of the '362 patent is discussed in detail in a paper presented at the Oct. 26-27, 1998 meeting of the Western States Section of the Combustion Institute (Paper No. 98F-36). The paper discloses a hypothetical process for using coal to power a gas turbine and reports on a series of preliminary experiments using an atmospheric pressure fluid bed of powdered chemically pure iron oxide (i.e., FeO/Fe2O3). The gas used to fluidize the bed can be switched from air to 5% SO2+95% N2 balance and back again. The experiments involved two basic process steps. In the first step, a bed fully oxidized to Fe2O3 was fluidized with the 5% SO2+95% N2 at a temperature of 857° C. A small amount of coal was then injected into this bed while the gases coming out of the bed were continuously analyzed. In a second step, the fluidizing gas was switched to air while continuing to analyze the gases coming from the bed.
The Combustion Institute paper also proposes a conceptual design for a process to use coal to power a gas turbine. As shown in FIG. 4 of the paper, the FeO/Fe2O3 catalyst is used as a fluidized powder which circulates between a first fluid bed fluidized with steam and a second bed fluidized with compressed air from the compressor section of a gas turbine. Within this bed, FeO is oxidized to Fe2O3—a strongly exothermic reaction that depletes the compressed air of oxygen while heating the air. The heated compressed air (now oxygen-depleted) can then be used to drive the expander section of a gas turbine. The Combustion Institute paper contemplates using pulverized coal as the main fuel source. See FIG. 4.
Thus, the prior art contains separate teachings of means for achieving the goal of oxidating coal to sequestration-ready CO2 and of means for achieving the goal of gasifying of coal to relatively pure hydrogen. The prior art, however, does teach, show or suggest means for achieving both these goals in the same process. A definite need exists for an improved method of burning (oxidizing) coal using unmixed combustion to produce sequestration-ready CO2, relatively pure hydrogen while at the same time creating a hot gas stream for use in generating electrical power by expansion through gas turbine engines.